Haloalkanes: Naming, Bonding and Preparation: Quick Revision

A brief note on Haloalkanes: Naming, Bonding and Preparation. Read it before a test. Try each question at the end before you open its answer.

Classes and names

• Haloalkane: X on an sp3 carbon. Haloarene: X on a ring carbon.
• Primary, secondary, tertiary: the carbon holding X touches one, two or three other carbons.
• Allylic: X on an sp3 carbon next to C=C. Benzylic: next to a ring. Vinylic and aryl: X on an sp2 carbon.
• Gem-dihalide: two X on one carbon. Vicinal: two X on neighbouring carbons.
• IUPAC: the halogen is a prefix (bromo, chloro, fluoro, iodo), in alphabetical order, with the lowest locants.
• Common names: methylene chloride $\text{CH}_{2}\text{Cl}_{2}$, chloroform $\text{CHCl}_{3}$, iodoform $\text{CHI}_{3}$, carbon tetrachloride $\text{CCl}_{4}$.

The C-X bond and the physical properties

• The bond is polar, with carbon $\delta^+$ and the halogen $\delta^-$.
• From C-F to C-I the bond gets longer and weaker. The dipole moment is highest for chlorine.
• The boiling point rises with halogen size and chain length, and falls with branching. It is above the alkane, and below the alcohol.
• Haloalkanes dissolve very little in water. Bromo and iodo compounds are denser than water.

Preparation

• From alcohols: HX (with ZnCl2 for HCl), $\text{PCl}_{5}$, $\text{PCl}_{3}$, red P with X2, or $\text{SOCl}_{2}$ (the cleanest, gaseous by-products).
• From alkanes: X2 with light gives mixtures. Chlorine is less selective than bromine, and iodine fails.
• From alkenes: HX by Markovnikov's rule. HBr with a peroxide gives the other product.
• Halogen exchange: Finkelstein (NaI, dry acetone) for iodides, Swarts (AgF and similar) for fluorides.

A worked example

2-Methylbutane is chlorinated in light. One primary, secondary and tertiary hydrogen react in the ratio $1 : 3.8 : 5.0$. How much 2-chloro-3-methylbutane forms?

1. Count. The secondary position has 2 hydrogen atoms.
2. Weights. Total weight is $6 \times 1.0 + 1 \times 5.0 + 2 \times 3.8 + 3 \times 1.0 = 21.6$.
3. Share. $\dfrac{2 \times 3.8}{21.6} \times 100 = 35.2\ \%$.

Practice with new numbers

A program to try

The program names the class of four haloalkanes and finds their halogen content.

Mistakes to avoid

• Counting the halogen as a carbon neighbour when classing a halide.
• Using sulphuric acid to make hydrogen iodide.
• Expecting a pure product from free radical halogenation.
• Forgetting the peroxide when the question says HBr adds against Markovnikov's rule.

Check yourself

1. Name $\text{CH}_{3}\text{CH}_{2}\text{CH}(\text{Cl})\text{CH}_{2}\text{CH}_{3}$ and say whether it is primary, secondary or tertiary.

2. Which bond is the weakest: C-Cl, C-Br or C-I?

3. Which reagent makes a chloroalkane from an alcohol with only gaseous by-products?

Sources

• NCERT, Chemistry Part II for Class XII, the chapter Haloalkanes and Haloarenes.
• OpenStax, Organic Chemistry, Chapter 10 (Organohalides).